The present invention relates to a battery separator and a lithium secondary battery. Particularly, the invention relates to a battery separator favorably employable for a lithium secondary battery and a lithium secondary battery utilizing the battery separator.
As a battery separator or a separator for electrolytic condensers, a porous polyolefin film which is produced by stretching a polyolefin film or by subjecting a polyolefin film containing micro-particles dispersed in the film to a procedure for removing the particles from the film is generally employed.
A recently developed non-aqueous lithium secondary battery of high quality requires a battery separator having improved characteristics. In more detail, a recently developed lithium secondary battery has a high energy density, a high electromotive force, and a low self discharge. In a representative lithium secondary battery, the negative electrode is made of a lithium metal, a metal alloy of lithium and other metal element, a carbonaceous material in which lithium ions are absorbed or intercalated, for instance, graphite, or an electro-conductive material doped with lithium ions, and a positive electrode is made of a fluorinated graphite represented by (CFx)n, a metal oxide (e.g., MnO2, V2O5, CuO, Ag2CrO4, TiO2, LiCoO2, LiNiO2, or LiMn2O4), a metal sulfate, or a metal oxide.
The representative lithium secondary battery further comprises a non-aqueous electrolytic solution in which a electrolyte such as LiPF6, LiBF4, LiClO4, LiCF3SO3, LiN(SO2CF3)2, or LiN(SO2C2F5)2 is dissolved in an organic solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, xcex3-butyrolactone, acetonitrile, 1,2-dimethoxyethane, or tetrahydrofuran.
The battery separator works to separate the positive electrode from the negative electrode so as to inhibit production of short circuit, while not disturbing the battery reaction.
Until now, various battery separators are developed. Examples of the known battery separators are given below.
A porous mono-layer film of thermoplastic resin such as polyethylene or polypropylene (see Japanese Patent Publications No. 46-40119, No. 55-32531, and No. 59-37292, Japanese Patent Provisional Publications No. 60-23954 and No. 2-75151, and U.S. Pat. No. 3,679,538).
A porous composite film comprising a number of porous polyethylene or polypropylene films (see Japanese Patent Provisional Publications No. 62-10857, No. 2-77108, No. 6-55629, No. 6-20671, and No. 7-307146).
A porous film of a resin having a high molecular weight (see Japanese Patent Provisional Publications No. 2-94356 and No. 3-105851).
A porous composite film having a support film of a thermoplastic resin or a non-woven fabric (see Japanese Patent Provisional Publications No. 3-245457 and No. 1-258358).
A porous film of polyolefin resin containing an inorganic filler in an amount of 50 to 500 weight parts per 100 weight parts of the resin material (see Japanese Patent Provisional Publication No. 62-167332).
A battery separator having a surface protective layer containing inorganic micro particles (see Japanese Patent Provisional Publication No. 11-80395).
In the course of studies on a battery separator, particularly a battery separator favorably employable for a lithium battery, the present inventors have noted that ills known battery separators cannot give satisfactory characteristics such that both of electric performance required for a battery separator and physical strengths are not simultaneously satisfied.
The present invention resides in a battery separator comprising at least one porous film in which 100 to 40,000 ppm of particles having a mean diameter in the range of 0.1 to 10 xcexcm are dispersed in a porous resin matrix, the particles comprising at least one metal oxide selected from the group consisting of silicon dioxide, aluminum oxide, magnesium oxide, zinc oxide and their complex compounds, namely, metal oxides containing at least two metal elements selected from the group consisting of Si, Al, Mg and Zn and having an oxidation potential of +4.5 V or more as compared with that of lithium.
The battery separator of the present invention can be favorably produced by a method of producing a porous film in which 100 to 40,000 ppm, particularly 100 to 5,000 ppm, of particles having a mean diameter of 0.1 to 10 xcexcm, particularly, 0.1 to 8 xcexcm, are dispersed in a porous resin matrix, the particles comprising at least one metal oxide selected from the group consisting of silicon dioxide, aluminum oxide, magnesium oxide, zinc oxide and their complex compounds, which comprises stretching a resin film in which the particles of metal oxide are dispersed.
The battery separator of the invention preferably comprises a combined multi-layer film composed of one porous resin (particularly, polyethylene) film containing no solid particles and a pair of the porous resin (particularly, polypropylene) films having the above-identified metal oxide particles in which the porous resin film containing no solid particles is placed between a pair of the porous resin films having the metal oxide particles.